Although fungi and parasites are important pathogens causing infectious diseases, their pathogenic mechanisms and virulence factors are not as well characterized as those of bacteria. Despite the relative lack of detailed mechanisms, the stages of pathogenesis and general mechanisms of virulence involved in disease production by these pathogens are similar to those of bacteria.

Fungal virulence

Pathogenic fungi can produce virulence factors that are similar to the bacterial virulence factors that have been discussed earlier in this chapter. In this section, we will look at the virulence factors associated with species of
Candida ,
Cryptococcus, Claviceps, and
Aspergillus .

Candida albicans is an opportunistic fungal pathogen and causative agent of oral thrush, vaginal yeast infections, and cutaneous candidiasis.
Candida produces adhesins (surface glycoproteins) that bind to the phospholipids of epithelial and endothelial cells. To assist in spread and tissue invasion,
Candida produces proteases and phospholipases (i.e., exoenzymes). One of these proteases degrades keratin, a structural protein found on epithelial cells, enhancing the ability of the fungus to invade host tissue. In animal studies, it has been shown that the addition of a protease inhibitor led to attenuation of
Candida infection.
K. Fallon et al. “Role of Aspartic Proteases in Disseminated
Candida albicans Infection in Mice.”
Infection and Immunity 65 no. 2 (1997):551–556. Similarly, the phospholipases can affect the integrity of host cell membranes to facilitate invasion.

The main virulence factor for
Cryptococcus , a fungus that causes pneumonia and meningitis, is capsule production. The polysaccharide glucuronoxylomannan is the principal constituent of the
Cryptococcus capsule. Similar to encapsulated bacterial cells, encapsulated
Cryptococcus cells are more resistant to phagocytosis than nonencapsulated
Cryptococcus , which are effectively phagocytosed and, therefore, less virulent.

Like some bacteria, many fungi produce exotoxins. Fungal toxins are called
mycotoxins .
Claviceps purpurea , a fungus that grows on rye and related grains, produces a mycotoxin called ergot toxin, an alkaloid responsible for the disease known as
ergotism . There are two forms of ergotism: gangrenous and convulsive. In
gangrenous ergotism , the ergot toxin causes vasoconstriction, resulting in improper blood flow to the extremities, eventually leading to gangrene. A famous outbreak of gangrenous ergotism occurred in Eastern Europe during the 5th century AD due to the consumption of rye contaminated with
C. purpurea . In
convulsive ergotism , the toxin targets the central nervous system, causing mania and hallucinations.

The mycotoxin
aflatoxin is a virulence factor produced by the fungus
Aspergillus , an
opportunistic pathogen that can enter the body via contaminated food or by inhalation. Inhalation of the fungus can lead to the chronic pulmonary disease
aspergillosis , characterized by fever, bloody sputum, and/or asthma. Aflatoxin acts in the host as both a
mutagen (a substance that causes mutations in DNA) and a
carcinogen (a substance involved in causing cancer), and has been associated with the development of
liver cancer . Aflatoxin has also been shown to cross the blood-placental barrier.
C.P. Wild et al. “In-utero exposure to aflatoxin in west Africa.”
Lancet 337 no. 8757 (1991):1602. A second mycotoxin produced by
Aspergillus is
gliotoxin . This toxin promotes virulence by inducing host cells to self-destruct and by evading the host’s immune response by inhibiting the function of phagocytic cells as well as the pro-inflammatory response. Like
Candida ,
Aspergillus also produces several proteases. One is
elastase , which breaks down the protein elastin found in the connective tissue of the lung, leading to the development of lung disease. Another is
catalase , an enzyme that protects the fungus from hydrogen peroxide produced by the immune system to destroy pathogens.

catalase test is done to differentiate between staph and strep.
as staph is catalase positive while strep is catalase negative then staph is differentiate further on coagulase positive and coagulase negative. staph aureus is coagulase positive while staph epidermidis is coagulase negative.

Muhammad

gram positive staph is further differentiated on sensitivity test and manitol salt fermentation test while gram positive strep is differentiated on hemolysis pattern